This application claims the priority of German patent document 101 40 498.0, filed Aug. 17, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method of suppressing jammer signals included in the signal received by a radar antennas of HPRF (high pulse repetition frequency) applications.
Interfering signals occurring in radar technology fall into two categories: wanted and unwanted. Wanted interfering signals include the jammer signal. Particularly in the military sector, a wanted interference (jammer signal) due to an electronic countermeasure may be desirable for protecting the target from being located and tracked. For this purpose, the target emits interfering and false signals which are superimposed on the useful signals at the radar unit, in order to generate faulty information concerning the target, and therefore inducing the enemy to take measures which are ineffective.
Unwanted interfering signals, also called clutter or noise signals, are generally signals which are generated by the reflection or scattering, for example, of energy beamed onto the surface of the earth.
The jammer signal is mainly captured by way of the side lobes of the radar antenna and is relatively strong on the basis of the 1/R2 law. In order to suppress this signalxe2x80x94also called side lobe jammerxe2x80x94, an adaptive side lobe canceller is used (S. P. Applebaum, xe2x80x9cAdaptive Arrayxe2x80x9d, IEEE Transactions of Antenna and Propagation, Vol. AP-24, No. 5, September 1976). This latter canceller is based on the existence of a sum channel, a difference channel and an auxiliary channel of the radar antenna, with the auxiliary channel having an isotropic antenna diagram, particularly 30-40 dB under the maximum of the major lobe of the antenna.
FIG. 1 illustrates a known technique for suppressing the side lobe jammer. The signal is received by the radar antenna in the time domain. In the further signal processing, the sum S, difference D and auxiliary channel G are subjected to a recursive clutter filter CF which suppresses the clutter signal. Subsequently, a side lobe cancellation for the three channels S,D,G is carried out by means of a side lobe canceller SC. Finally, in the case of the known technique, the signals of the three channels S, D, G are transformed in a Fast Fourier Transformation (FFT) from the time domain into the frequency range. For computing an FFT, normally signal processors with a floating-point arithmetic are used, whereby a more precise computation of the FFT can be carried out than by means of fixed-point processors.
However, the use of a high-expenditure recursive clutter filter when a subsequent FFT takes place has proven to be disadvantageous. In addition, this method is very sluggish and can react only slowly to changes of the side lobe jammer.
It is an object of the invention to provide a method for suppression of the side lobe jammer that is simple and fast.
This and other objects and advantages are achieved by the method according to the invention, in which a signal received by the radar antenna is first processed to form sum, difference and auxiliary channels which are then transformed from the time domain into the frequency, domain using a Fast Fourier Transformation. Subsequently, in the frequency domain, the jammer signal, particularly the side lobe jammer, is suppressed by means of side lobe cancellation.
This arrangement has the advantage that the processing of the signals in the side lobe canceller can take place more easily and faster in the frequency domain than in the time domain according to the prior art. Another advantage of the method according to the invention is the fact that complicated recursive clutter filters are required.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.